Optical reflectometric methods are widely employed for characterization of optical fiber links. Among these methods, the most commonly used approach is Optical Time-Domain Reflectometry (OTDR—also used to refer to the corresponding device), a diagnostic technique where light pulses are launched in an optical fiber link and the returning light, arising from backscattering and reflections along the fiber link, is detected and analyzed. Various “events” along the fiber link can be detected and characterized through a proper analysis of the returning light in the time domain and insertion loss of the fiber link under test can be characterized.
Multi-fiber array cables are widely employed nowadays in premises optical fiber cabling such as in data centers and other intrabuilding optical fiber networks, which require high connectivity density and versatile solutions. Multi-fiber array cables are mostly interconnected and connected to optical network equipment using MPO/MTP® connectors (MPO being the acronym for Multi-fiber Push-On/Pull-Off connectors and MTP a brand name). The most common MPO/MTP® connectors are configured in a 1×12 fiber array but 2×12 and 2×16 fiber arrays as well as other configurations are also possible.
Multi-fiber array cables are also commonly employed in combination with duplex optical cabling and optical fiber transition modules in premises optical fiber cabling. Multi-fiber array cabling may be arranged in duplex or parallel configurations. In a duplex multi-fiber array configuration, the optical fibers are arranged on the multi-fiber connectors such that on pairs of adjacent optical fibers, one fiber is used for transmitting and the other for receiving. Transmission and reception fibers therefore alternate on the connector. In a parallel multi-fiber array configuration, optical fibers used for transmitting and that used for receiving are physically separated into two groups of adjacent fibers on the multi-fiber array connectors. The position of receiving and transmitting optical fibers at a multi-fiber connector defines what is referred to in the industry as the polarity. Array system connectivity requires specific combinations of duplex patch cords, multi-fiber array cables and optical fiber transition modules in order to properly manage polarities in duplex or parallel configurations. The TIA/EIA-568-C.3 Standard conveniently defines guidelines for maintaining fiber polarity in array systems. Because various multi-fiber array configurations are possible (i.e. duplex configuration, one-plug parallel configuration, two-plug one-row parallel configuration and one-plug two-row configuration, see TIA/EIA-568-C.3), various multi-fiber array cable types are defined by the TIA/EIA-568-C.3 Standard, each having a specific 1×12 or 2×12 fiber arrangement. Type A, Type B and Type C (1×12) multi-fiber array cables as defined by the Standard are illustrated in FIGS. 1, 2 and 3 respectively. Type A cables as defined by the Standard are designed with a key inversion but no duplex pair twists between the input and output connectors. Type B cables are designed with no key inversion and no duplex pair twists. Type C cables are designed with a key inversion and with duplex pair twists. In one array systems, combinations of different cable types may be required depending on the multi-fiber array configuration employed. Hence, deployment mistakes can easily occur if the appropriate arrangement of cable types is not followed (e.g. some cables are mixed up) which result in improper fiber polarity at the multi-fiber array connections.
In order to attempt to prevent polarity mistakes, Corning™ has introduced a proprietary Universal Polarity Management Method that employs multi-fiber array cables at both ends of which a proprietary optical fiber transition module is connected. The same optical fiber transition module may be used at both ends. This proprietary optical fiber transition module allows polarity to be properly managed without duplex inversions either on duplex patchcords (see Method A as defined in the TIA/EIA-568-C.3 Standard) or within the multi-fiber array cable (Type C cables), and this irrespective of the deployed multi-fiber array configuration (i.e. duplex vs parallel configuration). But even when employing the Universal Polarity Management Method by Corning™, deployment mistakes may arise if the wrong cable types or optical fiber transition modules are installed.
Of course, other proprietary or custom multi-fiber array cabling systems may be used and may require characterization.
Characterization of optical fiber cabling is required to test network integrity and performance. Tier 1 characterization of multi-fiber array connections includes insertion loss measurements, cable length determination and fiber arrangement/cable type verification. These measurements may be performed with an Optical Loss Test Set (OLTS), but OLTS measurements are not sufficient for Tier 2 testing which, in addition to encompassing those measurement associated with Tier 1 testing, further require a complete characterization of the various elements along a fiber link. This complete characterization includes connector location, loss and reflection, splice location, loss and reflection, length and insertion loss of individual segments, as well as additional events that may cause insertion loss such as an unwanted optical fiber bend. OTDR measurements are therefore required for Tier 2 testing.
In order to properly characterize splice or other connection losses using OTDR measurements in an optical fiber link that may include concatenated singlemode optical fibers segments, the Telecommunications Industry Association (TIA) recommends the use of bi-directional OTDR analysis. Such analysis averages the results of single-ended OTDR measurements acquired in both directions of the fiber link under test (test procedure EIA/TIA FOTP-61 “Measurement of Fiber or Cable Attenuation Using an OTDR”), thereby removing ambiguities associated with single-end OTDR measurements. Small differences in fiber geometry between the different concatenated fiber segments in a link may induce small changes in the backscattering characteristics. As a consequence, this geometry mismatch between spliced or otherwise connected fibers may cause an apparent “gain” or a drop in the backscattered light of OTDR measurements, which introduces a bias in the insertion loss measurement. For example, a fiber connection may appear as a gain in the backscattered light due to a mismatch between the connected fibers. An OTDR measurement performed from the opposite end on the same fiber connection would conversely result in an overestimation of the connection loss. For this reason, the precision obtained with single-end OTDR measurements may not always be sufficient for Tier 1 and/or Tier 2 testing requiring characterization of optical fiber connections and/or overall insertion loss. The Telecommunications Industry Association (TIA) therefore recommends the use of bi-directional OTDR analysis to properly characterize optical fiber links. Bidirectional OTDR measurement also provides unambiguous continuity check.
OTDR methods exist for verifying the fiber arrangement/cable type of multi-fiber array cables (see for example WO 2013/181197 A1 to Collier et al.) but in order to perform bidirectional OTDR analysis with these methods, either two OTDR acquisition devices should be used, i.e. one at each end of the link under test, or a single OTDR acquisition device should be moved from one end to the other.